Electroconductivity-controlling agent for cationic electrodeposition coating composition and method for adjusting electroconductivity of cationic electrodeposition coating      composition therewith

ABSTRACT

The present invention is intended to provide a technology preventing decrease in an electroconductivity or an throwing power of a cationic electrodeposition coating composition having a lower solid content and/or a lower ash content. Therefore, the present invention relates to:
         an electroconductivity-controlling agent used for a low solid content type cationic electrodeposition coating composition having 0.5 to 9.0 wt % of solid content of the coating composition, which agent includes an amino group-containing compound having 500 to 20000 of molecular weight and 200 to 500 mmol/100 g of amine value, to adjust an electroconductivity of the composition to 900 to 2000 μS/cm;   a method for adjusting an electroconductivity of a cationic electrodeposition coating composition, which includes steps of:   adding an electroconductivity-controlling agent to a low solid content type cationic electrodeposition coating composition having 0.5 to 9.0 wt % of solid content of the coating composition, and   adjusting an electroconductivity of the cationic electrodeposition coating composition to 900 to 2000 μS/cm during the above step,
 
wherein the electroconductivity-controlling agent includes an amino group-containing compound having 200 to 500 mmol/100 g of amine value; and the like.

TECHNICAL FIELD

The present invention relates to an electroconductivity-controlling agent for a cationic electrodeposition coating composition and a method for adjusting an electroconductivity of a cationic electrodeposition coating composition therewith.

BACKGROUND ART

Cationic electrodeposition coating is widely utilized as a primer coating for an article which is large and has a complicated shape, especially for a vehicle body and the like, since the cationic electrodeposition coating can be formed to detailed portions, even if the article has a complicated shape, and the coating can be provided automatically and continuously. The cationic electrodeposition coating is carried out by immersing an article to be coated, as a cathode, into a cationic electrodeposition coating composition, and applying voltage.

The conventional cationic electrodeposition coating composition is a waterborne coating composition having about 20 wt % of solid content. Leaving the composition without stirring provides sedimentation of the component therein, such as pigment, which results in precipitation in the electrodeposition tank containing the composition. Generally, a cationic electrodeposition coating composition is circulated with a pump or stirred with a stirrer in order to prevent occurrence of the sedimentation.

The cation electrodeposition tank, however, is a large facility wherein a vehicle body can be immersed into a coating composition. Enormous costs are required for energy accompanied by the circulation or stirring the composition, a facility therefor, or maintenance thereof. Reducing or removing such circulation or stirring highly contributes to save energy upon cationic electrodeposition coating. Therefore, use of a cationic electrodeposition coating composition without sedimentation or with a little sedimentation, specifically, a cationic electrodeposition coating composition having a lower solid content or a lower ash content is effective. Such cationic electrodeposition coating composition has been investigated.

For example, JP-A-2004-231989 (Patent Literature 1) discloses an eco-friendly electrodeposition coating with a cationic electrodeposition coating composition having 3 to 10 wt % of pigment ash content and 5 to 12 wt % of solid content. This cationic electrodeposition coating composition is useful, since this composition has a small amount of sedimentation, and therefore cost for energy upon stirring or circulating is reduced. Adversely, the reduction in solid content of the coating composition lowers electroconductivity of the composition. It also deteriorates “throwing power” of the composition, herein the throwing power is an ability to form a coating film on an article to the detailed portions during the electrodeposition coating.

It is known to those skilled in the art that an appropriate adjustment of an electroconductivity of a coating composition can provide a desirable throwing power. As a patent literature, JP-A-2004-269627 (Patent Literature 2) disclosed a relation between an electroconductivity and a throwing power of a coating composition. The cationic electrodeposition coating composition described in the literature contains a sulfonium modified epoxy resin, and therefore the composition requires control of a film resistance.

An amine value of a base resin in a cationic electrodeposition coating composition has been investigated [see JP-A-2005-232397 (Patent Literature 3), JP-A-7-150079 (Patent Literature 4), etc]. The patent literature 3 discloses that it is desirable that an amine value of an urethane resin (base resin) is within a range of 20 to 60 mgKOH/g (i.e., 35.7 to 107.0 mmol/100 g). The patent literature 4 also discloses that it is desirable that an amine value of a cationic electrodepositable resin is within a range of 3 to 200 mgKOH/g (i.e., 5.3 to 356 mmol/100 g). These ranges of the amine value are conventional and essentially low.

-   Patent Literature 1: JP-A-2004-231989 -   Patent Literature 2: JP-A-2004-269627 -   Patent Literature 3: JP-A-2005-232397 -   Patent literature 4: JP-A-7-150079

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

A cationic electrodeposition coating composition having a lower solid content and/or a lower ash content inclines to have a lowered electroconductivity in comparison with a conventional cationic electrodeposition coating composition. The present invention provides a technology preventing decrease in electroconductivity of a cationic electrodeposition coating composition having a lower solid content and/or a lower ash content, and preventing decrease in throwing power wherewith.

Means for Solving Problem

Accordingly, the present invention provides an electroconductivity-controlling agent used for a low solid content type cationic electrodeposition coating composition having 0.5 to 9.0 wt % of solid content of the coating composition, which agent includes an amino group-containing compound having 500 to 20000 of molecular weight and 200 to 500 mmol/100 g of amine value, to adjust an electroconductivity of the composition to 900 to 2000 μS/cm. The cationic electrodeposition coating composition contains the present electroconductivity-controlling agent as an emulsion other than a cationic epoxy resin as a coating film forming component, a curing agent and a pigment, which emulsion may be actually added as a third component.

The amino group-containing compound employed as the electroconductivity-controlling agent may be an amine modified epoxy resin. The amine modified epoxy resin may be preferably an epoxy resin wherein an epoxy group has been modified with an amine compound.

Alternatively, the amino group-containing compound may be an amine modified acryl resin. The amine modified acryl resin may be preferably an acryl resin having an epoxy group modified with an amine compound.

The epoxy resin may be bisphenol type, t-butylcatechol type, phenol novolak type or cresol novolak type, and which may have 500 to 20000 of number average molecular weight.

The present invention also provides a low solid content type cationic electrodeposition coating composition having 0.5 to 9.0 wt % of solid content of the coating composition, which includes an electroconductivity-controlling agent comprising an amino group-containing compound having 200 to 500 mmol/100 g of amine value, and which composition has 900 to 2000 μS/cm of electroconductivity.

The present invention further provides a method for adjusting an electroconductivity of a cationic electrodeposition coating composition, which includes steps of:

adding an electroconductivity-controlling agent to a low solid content type cationic electrodeposition coating composition having 0.5 to 9.0 wt % of solid content of the coating composition and

adjusting an electroconductivity of the cationic electrodeposition coating composition to 900 to 2000 μS/cm during the above step,

wherein the electroconductivity-controlling agent includes an amino group-containing compound having 200 to 500 mmol/100 g of amine value.

The present invention yet further provides a method for supplying an electroconductivity-controlling agent to a cationic electrodeposition coating composition, which includes steps of:

supplying an electroconductivity-controlling agent to a low solid content type cationic electrodeposition coating composition having 0.5 to 9.0 wt % of solid content of the coating composition, and

adjusting an electroconductivity of the cationic electrodeposition coating composition to 900 to 2000 μS/cm during the above step,

wherein the electroconductivity-controlling agent includes an amino group-containing compound having 200 to 500 mmol/100 g of amine value.

EFFECT OF THE INVENTION

The present invention can solve problems associated with a cationic electrodeposition coating composition, which is a lower ash content type and/or a lower solid content type, such as decrease in electroconductivity of the cationic electrodeposition coating composition and decrease in throwing power therewith, by adding a certain electroconductivity-controlling agent for a cationic electrodeposition coating composition to the cationic electrodeposition coating composition.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view which shows an example of a box employed for evaluating the throwing power.

FIG. 2 is a cross sectional view which schematically shows one embodiment of the process for evaluating the throwing power.

EXPRESSION OF REFERENCE LETTERS

10: Box

11 to 14: Steel plates treated with zinc phosphate

15: Opening

20: Electrodeposition vessel

21: Electrodeposition coating composition

22: Counter electrode

Best Mode for Carrying Out the Invention

According to the present invention, an electroconductivity-controlling agent for a cationic electrodeposition coating composition includes an amino group-containing compound having 200 to 500 mmol/100 g of amine value. Any amino group-containing compound can be employed as an electroconductivity-controlling agent of the present invention for a cationic electrodeposition coating composition, if only the amino group-containing compound has the above-defined range of the amine value. Generally, an amine modified epoxy resin and an amine modified acryl resin are preferable for the agent. The present electroconductivity-controlling agent for a cationic electrodeposition coating composition, if necessary, may be neutralized with an acid. The amine value is preferably 250 to 450 mmol/100 g, most preferably 300 to 400 mmol/100 g. If the amine value is less than 200 mmol/100 g, the necessary amount of the agent to be added is increased in order to optimize electroconductivity of a liquid cationic electrodeposition coating composition having a lower solid content, which may induce an inferior corrosion resistance. If the amine value is more than 500 mmol/100 g, there may be some problems such as reduction in depositability and undesirable throwing power. In this case, compatibility to a zinc-steel plate may also be inclined.

The amino group-containing compound employed in the present invention as an electroconductivity-controlling agent for a cationic electrodeposition coating composition includes an amino group-containing compound having a low molecular weight or a high molecular weight. Generally, an amino group-containing compound having a high molecular weight such as an amine modified epoxy resin and an amine modified acryl resin may be used. An example of the amino group-containing compound having a low molecular weight includes monoethanolamine, diethanolamine, dimethylbutylamine, etc.

The present invention preferably employs an amino group-containing compound having a high molecular weight, most preferably, an amine modified epoxy resin and an amine modified acryl resin. The amine modified epoxy resin is obtainable by modifying an epoxy resin, i.e., an epoxy group therein, with an amine compound. A conventional epoxy resin may be used, which is preferably an epoxy resin having 500 to 20000 of molecular weight, such as bisphenol type epoxy resin, t-butylcatechol type epoxy resin, phenol novolak type epoxy resin and cresol novolak type epoxy resin. Among these epoxy resins, the phenol novolak type epoxy resin and cresol novolak type epoxy resin are particularly desirable. Particularly, it is noted that these epoxy resins are commercially available, and for example, include DEN-438, phenol novolak type epoxy resin, available from the Dow Chemical Company, Japan; YDCN-703, cresol novolak type epoxy resin, available form Tohto Kasei Co., Ltd.; etc.

The epoxy resin may be modified with a resin such as a polyester polyol, a polyether polyol and a mono-functional alkyl phenol. Alternatively, the epoxy resin may be subjected to a chain extension including a reaction of an epoxy group therein with a diol or a dicarboxylic acid.

An example of the amine modified acryl resin includes a homopolymer of dimethylaminoethyl methacrylate, which is an amino group-containing monomer, without any modification, or a copolymer of dimethylaminoethyl methacrylate with other polymerizable monomer, without any modification, and a modified homopolymer of glycidyl methacrylate wherein the glycidyl group is modified with an amine compound, or a modified copolymer of glycidyl methacrylate with other polymerizable monomer wherein the glycidyl group is modified with an amine compound.

The compound which can introduce an amino group into an epoxy resin or an acryl resin having an epoxy group includes primary amine, secondary amine, tertiary amine, such as butylamine, octylamine, diethylamine, dibutylamine, dimethylbutylamine, monoethanolamine, diethanolamine, N-methyl-ethanolamine, triethylamine hydrochloride, N,N-dimethylethanolamine acetate, a mixture of diethyldisulfide and acetic acid, and secondary amine, which is a blocked primary amine, such as diketimine of aminoethylethanolamine and diketimine of diethylhydroamine, etc. One or more amines are available.

As described above, each of the amine modified epoxy resin and the amine modified acryl resin preferably has 500 to 20000 of number average molecular weight. If the number average molecular weight is less than 500, corrosion resistance may be decreased, throwing power may be decreased, and compatibility to a zinc-steel plate may be inclined, but these reasons are unknown. If the number average molecular weight is more than 20000, a poor finished appearance may result.

The above-described electroconductivity-controlling agent according to the present invention is applicable to a cationic electrodeposition coating composition, which includes, but is not limited to, a lower solid content type cationic electrodeposition coating composition having 0.5 to 9.0 wt % of solid content. The present electroconductivity-controlling agent is also applicable to a conventional cationic electrodeposition coating composition having about 20 wt % of solid content. In the circumstances the electroconductivity of the conventional cationic electrodeposition coating composition may be lowered, the electrodeposition coating therewith may provide an insufficient throwing power. Regarding such insufficiencies, addition of the present electroconductivity-controlling agent to the conventional cationic electrodeposition coating composition allows the electroconductivity to be controlled within an appropriate range, which results in a provision of an assurable sufficient throwing power.

The amine modified epoxy resin or the amine modified acryl resin which is to be employed in the present invention may be neutralized with a neutralizing acid in advance. The neutralizing acid includes an inorganic acid and an organic acid, such as hydrochloric acid, nitric acid, phosphoric acid, sulfamic acid, formic acid, acetic acid, lactic acid, etc.

Electrodeposition Coating Composition

Regarding the present electroconductivity-controlling agent for a cationic electrodeposition coating composition, adjustment of an amount of the agent to be added to the cationic electrodeposition coating composition can preferably control electroconductivity of the electrodeposition coating composition. The cationic electrodeposition coating composition includes a cationic epoxy resin and a curing agent, and if necessary, a pigment and/or an additive. These components are respectively described hereinafter.

Cationic Epoxy Resin (Cationic Modified Epoxy Resin as a Coating Film Forming Component)

The cationic epoxy resin which may be employed in the present invention includes an epoxy resin modified with an amine. The cationic epoxy resin is typically produced by opening all of the epoxy rings of a bisphenol type epoxy resin with an active hydrogen compound which can introduce a cationic group, or by opening a portion of epoxy rings with other active hydrogen compound and then opening the residual epoxy rings with an active hydrogen compound which can introduce a cationic group. The cationic epoxy resin included in the cationic electrodeposition coating composition preferably has 50 to 200 mmol/100 g of amine value, which is smaller than that of the electroconductivity-controlling agent (i.e., 200 to 500 mmol/100 g). If the amine value is less than 50 mmol/100 g, it is difficult to ensure the dispersibility of the cationic modified epoxy resin to water. If the amine value is more than 200 mmol/100 g, water resistance of the resulting coating film may be deteriorated. Therefore, these cases are not preferable.

A typical example of the bisphenol type epoxy resin includes a bisphenol A type epoxy resin and a bisphenol F type epoxy resin. The commercially available product of the former includes Epikote 828 (manufactured by Yuka-Shell Epoxy Co., Ltd., epoxy equivalent: 180 to 190), Epikote 1001 (the same manufacturer, epoxy equivalent: 450 to 500), Epikote 1010 (the same manufacturer, epoxy equivalent: 3000 to 4000) and the like, and the commercially available product of the latter includes Epikote 807 (the same manufacturer, epoxy equivalent: 170) and the like.

An epoxy resin containing an oxazolidone ring which is represented by the following formula and disclosed in JP-A-5-306327:

wherein R means a residual group formed by removing a glycidyloxy group of a diglycidylepoxy compound, R′ means a residual group formed by removing an isocyanate group of a diisocyanate compound, and n means a positive integer, may be used as the cationic epoxy resin. This is because the resulting coating layer is superior in heat resistance and corrosion resistance.

An example of the method for introducing an oxazolidone ring into an epoxy resin includes reacting a polyepoxide with a blocked polyisocyanate which has been blocked with a lower alcohol such as methanol, in the presence of a basic catalyst, with heating and keeping its temperature, and distilling off a lower alcohol as a by-product from the system to give the product.

Such epoxy resin may be modified with an appropriate resin such as polyester polyol, polyether polyol and monofunctional alkylphenol. Furthermore, the epoxy resin can extend its chain by utilizing the reaction of an epoxy group with a diol or a dicarboxylic acid.

It is desirable that the ring of the epoxy resin is opened with an active hydrogen compound so that an amine value is 50 to 200 mmol/100 g, after ring opening, and the primary amino group occupies more preferably 5 to 50% therein.

The active hydrogen compound which can introduce a cationic group includes the acid salts of primary amine, secondary amine and tertiary amine, sulfide and an acid mixture. The acid salts of primary amine, secondary amine or/and tertiary amine(s) are used as the active hydrogen compound which can introduce a cationic group in order to prepare an epoxy resin containing primary amino, secondary amino or/and tertiary amino group(s).

Specific examples include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methyl-ethanolamine, triethylamine hydrochloride, N,N-dimethyl-ethanolamine acetate, a mixture of diethyldisulfide and acetic acid, and secondary amine, which is a blocked primary amine, such as ketimine of aminoethylethanolamine and diketimine of diethylenetriamine, etc. One or more amines are available.

Curing Agent

As a curing agent to be employed in the present invention, a blocked polyisocyanate, which is a polyisocyanate blocked with a blocking agent, is preferable. The polyisocyanate, as used herein, means a compound having 2 or more of isocyanate groups in a molecule. An example of the polyisocyanate includes any type of polyisocyanates, such as an aliphatic type, an alicyclic type, an aromatic type, an aromatic-aliphatic type.

Specific example of the polyisocyanate includes aromatic diisocyanates such as tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), p-phenylene diisocyanate and naphthalene diisocyanate; aliphatic diisocyanates having 3 to 12 carbon atoms, such as hexamethylene diisocyanate (HDI), 2,2,4-trimethylhexane diisocyanate and lysine diisocyanate; alicyclic diisocyanates having 5 to 18 carbon atoms, such as 1,4-cyclohexane diisocyanate (CDI), isophorone diisocyanate (IPDI), 4,4′-dicyclohexylmethane diisocyanate (hydrogenated MDI), methylcyclohexane diisocyanate, isopropylidene dicyclohexyl-4,4′-diisocyanate and 1,3-diisocyanatomethylcyclohexane (hydrogenated XDI), hydrogenated TDI and 2,5- or 2,6-bis(isocyanatomethyl)-bicyclo[2.2.1]heptane (also called as norbornane diisocyanate); aliphatic diisocyanates having an aromatic ring, such as xylylene diisocyanate (XDI) and tetramethylxylylene diisocyanate (TMXDI); the modified products of these diisocyanates (urethanated product, carbodiimides, urethodione, urethoimine, biuret and/or isocyanurate modified product), etc. These can be used alone or 2 or more thereof can be used in combination.

An adduct or a prepolymer which is obtained by reacting a polyisocyanate with a polyvalent alcohol such as ethylene glycol, propylene glycol, trimethylolpropane or hexanetriol at an NCO/OH ratio of 2 or more may be also used as a curing agent.

As a polyisocyanate, aliphatic polyisocyanate and alicyclic polyisocyanate are preferable, since the resulting coating film has superior weather-resistance.

Specific preferable examples of the aliphatic polyisocyanate and the alicyclic polyisocyanate include hexamethylene diisocyanate, hydrogenated TDI, hydrogenated MDI, hydrogenated XDI, IPDI, norbornane diisocyanate, and dimer (biuret) and trimer (isocyanurate) thereof, etc.

The blocking agent is added to a polyisocyanate group, stable at ambient temperature, but can regenerate a free isocyanate group when it is heated to the dissociation temperature or more.

In the case the curing process is carried out desirably at lower temperature (e.g., no more than 160° C.), there can be preferably used, as a blocking agent, lactam type blocking agents such as ε-caprolactam, δ-valerolactam, γ-butyrolactam and β-propiolactam, and oxime type blocking agents such as formaldoxime, acetoaldoxime, acetoxime, methyl ethyl ketoxime, diacetyl monoxime and cyclohexane oxime.

Content of a binder containing a cationic epoxy resin and a curing agent is generally 25 to 85 wt %, preferably 40 to 70 wt % relative to total solid content of the electrodeposition coating composition.

Pigment

The electrodeposition coating composition to be employed in the present invention may include pigment(s) which have been conventionally used. The example of the pigment, which may be used, includes conventionally used inorganic pigments, for example, coloring pigments such as titanium white, carbon black and colcothar; filler pigments such as kaoline, talc, aluminum silicate, calcium carbonate, mica and clay; corrosion resistant pigments such as zinc phosphate, iron phosphate, aluminum phosphate, calcium phosphate, zinc phosphite, zinc cyanide, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, calcium molybdate and aluminum phosphomolybdate, aluminum zinc phosphomolybdate, bismuth hydroxide, bismuth oxide, basic bismuth carbonate, bismuth nitrate, bismuth benzoate, bismuth citrate, bismuth silicate, etc.

Content of the pigment is generally 1 to 35 wt %, preferably 10 to 30 wt % relative to total solid content of the electrodeposition coating composition.

Pigment-Dispersed Paste

When the pigment is used as a component of the electrodeposition coating composition, the pigment is generally preliminarily dispersed in an aqueous medium, together with a resin called as a pigment-dispersing resin, at a high concentration to be a paste. Since the pigment is in a form of powder, it is difficult to disperse the pigment by one step to a uniform low concentration state which is used for the electrodeposition coating composition. In general, such paste is called as pigment-dispersed paste.

The pigment-dispersed paste can be prepared by dispersing a pigment in an aqueous medium together with a pigment-dispersing resin varnish. Generally, the pigment-dispersing resin varnish includes a cationic or nonionic surfactant having a low molecular weight and a cationic polymer such as a modified epoxy resin having a quaternary ammonium group and/or a tertiary sulfonium group. The aqueous medium includes ion-exchanged water, water containing a small amount of alcohol(s), etc. Generally, solid content of the pigment-dispersing resin varnish in the pigment-dispersed paste is 5 to 40 parts by weight and solid content of the pigment in the pigment-dispersed paste is 10 to 30 parts by weight.

10 to 1000 parts by weight of the above-described pigment-dispersing resin varnish and pigment are mixed relative to 100 parts by weight of the solid resin content. Subsequently, the pigment is dispersed in the mixture in a conventional dispersing apparatus such as a ball mill and a sand-grinding mill to produce a pigment-dispersed paste wherein the pigment has a given uniform particle size.

The above-described cationic electrodeposition coating composition according to the present invention must have 0.5 to 9.0 wt % of solid content of the coating composition. If the solid content of the coating composition is less than 0.5 wt %, there is no resulting of a cationic electrodeposition coating. On the other hand, if the solid content of the coating composition is more than 9.0 wt %, there is precipitation of the pigment component contained in the cationic electrodeposition coating composition which is left alone without stirring. Therefore, these cases are not preferable.

Preparation of Electrodeposition Coating Composition

The electrodeposition coating composition can be prepared by dispersing a cationic epoxy resin, a curing agent and a pigment-dispersed paste into an aqueous medium. Generally, the aqueous medium may include a neutralizing agent to improve dispersibility of the cationic epoxy resin. The neutralizing agent includes an inorganic acid and an organic acid, such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid and lactic acid. The amount of the neutralizing agent to be used is an amount sufficient to achieve at least 20%, preferably 30 to 60% of neutralization ratio.

The amount of the curing agent to be used should be an amount sufficient to react with primary, secondary and/or tertiary amino group(s) or active hydrogen-containing functional group(s) such as hydroxyl group in the cationic epoxy resin during a curing process in order to provide an excellent cured coating film. Generally, a range of the weight ratio of the cationic epoxy resin/the curing agent (as basis of solid content) is in a range of 90/10 to 50/50, preferably 80/20 to 65/35.

The electrodeposition coating composition may include, as a catalyst, a tin compound such as dibutyltin dilaurate and dibutyltin oxide, or an urethane cleavage catalyst which have been conventionally used. Since these catalysts are, preferably, substantially free from lead compounds, the amount of the catalyst to be used is preferably 0.1 to 5 wt % relative to the amount of the blocked polyisocyanate compound.

The electrodeposition coating composition may include an additive which is conventionally used for coating compositions, such as a water-compatible organic solvent, a surfactant, an antioxidant, an UV absorbing agent and a pigment.

The present cationic electrodeposition coating composition is not particularly limited, if only the coating composition includes above-described essential components. The cationic electrodeposition coating composition, wherein the electroconductivity-controlling agent according to the present invention significantly effects, is a lower solid content type. The present cationic electrodeposition coating composition may be a lower ash content type.

The cationic electrodeposition coating composition, which is a lower solid content type, has a lower solid content, particularly 0.5 to 9 wt %, more particularly 3 to 9 wt %, than that of the conventional coating composition (i.e., about 20 wt %). If the solid content is less than 0.5 wt %, there is undesirably precipitation of the pigment component, without stirring, in the composition. On the other hand, if the solid content is more than 9 wt %, which is acceptable, addition of the electroconductivity-controlling agent in order to adjust the electroconductivity of such cationic electrodeposition coating composition may be in vain.

A method for reducing solid content of a cationic electrodeposition coating composition includes reduction of pigment content in the coating composition, wherein an ash content is reduced, herein, ash content is calculated by the following formula: [(weight of solid content of ash which is a residue after combustion of the coating composition)/(weight of solid content of the coating composition)]×100. Therefore, the present invention may employ a cationic electrodeposition coating composition which is a lower ash content type. The conventional cationic electrodeposition coating composition has 15 to 40 wt % of ash content. Therefore, the lower ash content type cationic electrodeposition coating composition preferably has 2 to 7 wt %, more preferably 3 to 5 wt % of ash content.

It is preferably that the article to be subjected to the electrodeposition coating and to be coated with the electrodeposition coating composition is a conductor previously subjected to a surface treatment such as a zinc phosphate treatment by an immersing or splaying method, etc. Alternatively, the surface of the article may be untreated. The conductor, as herein used, means any material which can be a cathode upon electrodeposition coating and which is preferably, but is not particularly limited to, a metal substrate.

Conditions for the electrodeposition coating are similar to those conventionally used in any electrodeposition coating. The applied voltage may be varied significantly in a range of 1 volt to a few hundreds volt. The current density is generally about 10 ampere/m² to 160 ampere/m². The current density tends to be decreased during electrodeposition coating.

After electrodeposition coating according to the present method, the resulting coating is subjected to a conventional baking process at elevated temperature, which includes baking in a stove or baking oven or under infrared heat lamp. Generally, the baking temperature may be varied in a range of about 140° C. to about 180° C. The article coated with the cationic electrodeposition coating composition according to the present invention is finally rinsed with water, dried and baked to form a cured electrodeposition coating film thereon. The present coating process may be completed.

Adjustment of Electroconductivity (Conductivity)

According to the present invention, the above-described electroconductivity-controlling agent is added to a liquid cationic electrodeposition coating composition to ensure electroconductivity of the liquid coating composition. As described above, the cationic electrodeposition coating composition, which has a lower solid content compared with that of the conventional liquid cationic electrodeposition coating composition (i.e., about 20 wt % of solid content), is used to have an insufficient electroconductivity. Therefore, addition of the present certain electroconductivity-controlling agent to the cationic electrodeposition coating composition can compensate the insufficiency. Increase in the amine value of the cationic modified epoxy resin as a coating film forming component can adjust the electroconductivity of the composition to an appropriate range in order to ensure the throwing power of the composition. However, if the amine value of the cationic modified epoxy resin is more than 200 mmol/100 g, the water resistance of the resulting coating film may be deteriorated. Such conditions are not preferable. The necessary electroconductivity for the desired throwing power is 900 to 2000 μS/cm. The addition of the present electroconductivity-controlling agent to a liquid cationic electrodeposition coating composition, which is a lower solid content type, can control and adjust the electroconductivity of the electrodeposition coating composition in a desired range. Preferable lower limit of the electroconductivity is 1000 μS/cm and preferable upper limit is 1800 μS/cm. If the electroconductivity is less than 900 μS/cm, there is a problem that the desired throwing power is not realized. If the electroconductivity is more than 2000 μS/cm, there is a problem that deficiencies on the coating film formed on a zinc-steel plate (i.e., so-called gas-pinholes) are frequent. Herein, the electroconductivity can be measured with a commercially available electroconductivity meter at 25° C. of the liquid composition temperature.

An amount of the present electroconductivity-controlling agent added to the cationic electrodeposition coating composition is not particularly limited, if only the desired electroconductivity is realized. Specific example of the amount is 0.5 to 30 wt %, preferably 1 to 30 wt %, more preferably 1 to 15 wt % relative to the solid contents of the coating composition. If the amount is less than 0.5 wt %, which is acceptable, an insufficient electroconductivity may be realized. Alternatively, if the amount is more than 50 wt %, which may be also acceptable, the increase in the electroconductivity is not proportional to the added amount.

As described above, the lower solid content type cationic electrodeposition coating composition having the desirably adjusted electroconductivity may be a lower ash and solid content type cationic electrodeposition coating composition, and preferably which may have a ensured throwing power. Even if such cationic electrodeposition coating composition is used in a coating process wherein many articles are coated continuously, it is necessary to supply a coating film forming component to the tank containing the cationic electrodeposition coating composition. In such case, the electroconductivity of the cationic electrodeposition coating composition in the tank may be undesirably deviated form the range of 900 to 2000 μS/cm, which is desired in the present invention. If the electroconductivity is no more than 900 μS/cm, the present electroconductivity-controlling agent may be further added to the tank containing the cationic electrodeposition coating composition in order to maintain the solid content within the range of 0.5 to 9.0 wt % and to adjust the electroconductivity of the cationic electrodeposition coating composition in the tank within the range of 900 to 2000 μS/cm.

Examples

The present invention is further described in detail in accordance with the following Examples. Those skilled in the art will appreciate that the present invention is not limited to these Examples. In the Examples, “part(s)” and “%” are based on weight unless otherwise specified.

Example A-1

295 Parts of methyl isobutyl ketone (hereinafter, which is abbreviated as MIBK), 37.5 parts of methylethanolamine and 52.5 parts of diethanolamine were charged into a flask equipped with a reflux condenser and a stirrer. The temperature of the mixture was kept at 100° C. with stirring. 205 Parts of a cresol novolak type epoxy resin, which is available form Tohto Kasei Co., Ltd., under the product name of YDCN-703, were gradually added to the mixture. After the complete addition of the resin, the reaction took place for 3 hours. The resulting amino modified resin has 2100 of molecular weight and 340 mmol/100 g of amine value (MEQ(B)).

Example A-2

5.5 Parts of formic acid and 1254.5 parts of deionized water were added to 140 parts of the amino modified resin solution prepared in Example A-1. The temperature of the mixture was kept at 80° C. with stirring for 30 minutes. The organic solvent was removed in vacuo to give electroconductivity-controlling agent A (solid content: 7.0%) for a liquid composition.

Example B-1

255 Parts of MIBK and 75 parts of methylethanolamine were charged into a flask equipped with a reflux condenser and a stirrer. The temperature of the mixture was kept at 100° C. with stirring. 180 Parts of a phenol novolak type epoxy resin, which is available form the Dow Chemical Company, Japan, under the product name of DEN-438, were gradually added to the mixture. After the complete addition of the resin, the reaction took place for 3 hours. The resulting amino modified resin has 1000 of molecular weight and 390 mmol/100 g of amine value (MEQ(B)).

Example B-2

14 Parts of sulfamic acid and 1247 parts of deionized water were added to 140 parts of the amino modified resin solution prepared in Example B-1. The temperature of the mixture was kept at 80° C. with stirring for 30 minutes. The organic solvent was removed in vacuo to give electroconductivity-controlling agent B (solid content: 7.0%) for a liquid composition.

Example C-1

50 Parts of methyl isobutyl ketone (MIBK) were charged into a flask equipped with a reflux condenser, a nitrogen introducing tube, a dropping funnel and a stirrer. The temperature of the mixture was kept at 100° C. with stirring. A mixture of 100 parts of glycidyl methacrylate and 2 parts of azobisisobutyronitrile (AIBN) was constantly added dropwise into the flask for 2 hours by using the dropping funnel. The temperature of the mixture was kept at 100° C. with stirring for 30 minutes. Subsequently, a mixture of 52.5 parts of MIBK and 0.5 part of AIBN was added dropwise into the flask over 1 hour. The reaction took place for another 1 hour with stirring. The reaction was quenched.

Example C-2

47.5 Parts of MIBK and 52.8 parts of methylethanolamine were charged into a flask equipped with a reflux condenser and a stirrer. The temperature of the mixture was kept at 100° C. with stirring. 205 Parts of the reaction mixture prepared in Example C-1 were gradually added to the mixture. After the complete addition of the reaction mixture, the reaction took place for 3 hours. The resulting amino modified resin has 9800 of molecular weight and 450 mmol/100 g of amine value (MEQ(B)).

Example C-3

25.2 Parts of lactic acid and 1234.8 parts of deionized water were added to 140 parts of the amino modified resin solution prepared in Example C-2. The temperature of the mixture was kept at 80° C. with stirring for 30 minutes. The organic solvent was removed in vacuo to give electroconductivity-controlling agent C (solid content: 7.0%) for a liquid composition.

Comparative Example D

463.4 Parts of deionized water and 13.5 parts of formic acid were added into a glass beaker. The mixture was stirred. 23.1 Parts of dimethylethanolamine (molecular weight: 89) were gradually added to the mixture with stirring to give electroconductivity-controlling agent D (active ingredient content: 7%; amine value (MEQ(B)) of the active ingredient: 740 mmol/100 g) for a liquid composition.

Preparation Example 1 Preparation of Cationic Electrodeposition Coating Composition Preparation Example 1-1 Preparation of Amine Modified Epoxy Resin

92 Parts of 2,4-/2,6-tolylenediisocyanate (2,4-form/2,6-form=8/2 as weight ratio), 95 parts of methyl isobutyl ketone (hereinafter, which is abbreviated as MIBK) and 0.5 part of dibutyltin dilaurate were charged into a flask equipped with a stirrer, a condenser, a nitrogen introducing tube, a thermometer and a dropping funnel. 21 Parts of methanol was added dropwise to the reaction mixture with stirring. The reaction proceeded at room temperature. The reaction temperature was raised to 60° C. due to the resulting heat of the reaction. The reaction was continued for 30 minutes. 50 Parts of ethylene glycol mono-2-ethylhexyl ether was added dropwise to the reaction mixture by using the dropping funnel. 53 Parts of propyleneoxide (5 mol) adduct of bisphenol A were further added to the reaction mixture. The reaction predominantly took place at in a range of 60 to 65° C. The reaction was continued until the absorption peak identified with the isocyanate group was disappeared upon IR spectrum measurement.

Subsequently, 365 parts of an epoxy resin having 188 of epoxy equivalent, which had been synthesized from bisphenol A and epichlorohydrin according to a known method, were added to the reaction mixture. The reaction temperature was raised to 125° C. 1.0 Part of benzyldimethylamine was added to the mixture. The reaction took place at 130° C. to adjust the epoxy equivalent to 410.

Subsequently, 61 parts of bisphenol A and 33 parts of octyl acid were added to the mixture. The reaction took place at 120° C. to allow the epoxy equivalent to be 1190, and then the reaction mixture was cooled. 11 Parts of diethanolamine, 24 parts of N-ethyl-ethanolamine and 25 parts of 79 wt % of MIBK solution of a ketimine of aminoethylethanolamine were added to the reaction mixture. The reaction took place at 110° C. for 2 hours. The reaction mixture was diluted with MIBK (non-volatile content: 80%). The resulted amine modified epoxy resin has 80% of solid resin content.

Preparation Example 1-2 Preparation of Blocked Isocyanate Curing Agent

1250 Parts of diphenylmethanediisocyanate and 266.4 parts of MIBK were charged into a reaction vessel. This mixture was headed to 80° C. 2.5 Parts of dibutyltin dilaurate were added to the mixture. 226 Parts of ε-caprolactam were dissolved in 944 parts of butyl cellosolve to give a solution. The solution was added dropwise to the mixture at 80° C. over 2 hours. The reaction mixture was further heated at 100° C. for 4 hours. It was confirmed that the absorption peak identified with the isocyanate group was disappeared upon IR spectrum measurement. The reaction mixture was left to be cooled. 336.1 Parts of MIBK were added to the reaction mixture to give a blocked isocyanate curing agent (glass transition temperature: 0° C.).

Preparation Example 1-3 Preparation of Pigment Dispersing Resin

222.0 Parts of isophorone diisocyanate (hereinafter abbreviated as IPDI) were added into a reaction vessel equipped with a stirrer, a condenser, a nitrogen introducing tube and a thermometer. 39.1 Parts of MIBK were added into the vessel to dilute IPDI, and then 0.2 part of dibutyltin dilaurate was added to the mixture. Subsequently, temperature was raised to 50° C. and 131.5 parts of 2-ethylhexanol were added dropwise over 2 hours, under dried nitrogen atmosphere, with stirring. The reaction temperature was kept at 50° C. with appropriately cooling to give a 2-ethylhexanol-halfblocked IPDI (solid resin content: 90.0%).

Subsequently, 87.2 parts of dimethylethanolamine, 117.6 parts of 75% aqueous solution of lactic acid, and 39.2 parts of ethylene glycol monobutyl ether were added in this order into an appropriate reaction vessel. The reaction mixture was stirred for about 30 minutes at 65° C. to give a quarterizing agent.

Subsequently, 710.0 parts of EPON 829 (bisphenol A type epoxy resin available from Shell Chemical Company; epoxy equivalent: 193 to 203) and 289.6 parts of bisphenol A were charged into an appropriate reaction vessel. The reaction mixture was heated to 150 to 160° C. under a nitrogen atmosphere. An initial exothermic reaction took place. The reaction mixture was maintained at 150 to 160° C. to keep the reaction for about 1 hour, and then cooled to 120° C. 498.8 Parts of the previously prepared 2-ethylhexanol-halfblocked IPDI (as MIBK solution) were added to the reaction mixture.

The reaction mixture was maintained at 110 to 120° C. for about 1 hour. Subsequently, 463.4 parts of ethylene glycol monobutyl ether were added. The mixture was cooled to 85 to. 95° C. After homogenization, 196.7 parts of the previously prepared quaternarizing agent were added to the reaction mixture. The reaction mixture was maintained at 85 to 95° C. to adjust the acid value to 1. 964 Parts of deionized water was added to the reaction mixture to quench the quaternarization of the epoxy-bisphenol A resin to give a pigment dispersing resin having a quaternary ammonium salt structure (solid resin content: 50%).

Preparation Example 1-4 Preparation of Pigment Dispersed Paste

100 Parts of the pigment dispersing resin prepared in Preparation Example 1-3, 100.0 parts of titanium dioxide and 100.0 parts of ion-exchanged water were charged into a sand-grinding mill. The pigment was dispersed to adjust the particle size to no more than 10 μm to give a pigment dispersed paste (solid content: 50%).

Preparation Example 1-5 Preparation of Emulsion

The amine modified epoxy resin prepared in Preparation Example 1-1 and the blocked isocyanate curing agent prepared in Preparation Example 1-2 were homogeneously mixed (epoxy resin/curing agent=80/20 as basis of solid content). Glacial acetic acid was added to the mixture so that the ratio (MEQ(A)): (mg equivalent of the acid)/(100 g of the solid content of the resin) was 30. Ion-exchanged water was slowly added to the mixture to dilute the mixture. MIBK was removed in vacuo to give an emulsion (solid content: 36%).

Comparative Example 1

319 Parts of the emulsion prepared in Preparation Example 1-5, 133 parts of the pigment dispersed paste, 543 parts of ion-exchanged water, 2 parts of 10% aqueous solution of cerium acetate, and 3 parts of dibutyltin oxide were mixed to give electrodeposition coating composition F (solid content: 20%). The solid content of the cationic electrodeposition coating composition includes the pigment at concentration of 23 wt %. Herein, the solid content of the coating composition can be calculated from the following formula (according to JIS K 5601): [(mass of the residue after heating the composition at 180° C. for 30 minutes)/(mass of the original composition)]×100(%). The resulted electrodeposition coating composition F was used as Comparative Example 1 as it was. The electroconductivity of the liquid composition was 1600 μS/cm.

Comparative Example 2

158 Parts of the emulsion prepared in Preparation Example 1-5, 8 parts of the pigment dispersed paste, 831 parts of ion-exchanged water, 2 parts of 10% aqueous solution of cerium acetate, and 1 part of dibutyltin oxide were mixed to give electrodeposition coating composition G (solid content: 7%). The pigment concentration was 5 wt %. The resulted electrodeposition coating composition G was used as Comparative Example 2 as it was. The electroconductivity of the liquid composition was 890 μS/cm.

Example 1

6 Parts of the electroconductivity-controlling agent A prepared in Example A-2 were added to 1000 parts of the previously prepared electrodeposition coating composition G to adjust the electroconductivity of the composition to 1200 μS/cm to give electrodeposition coating composition H. The electrodeposition coating composition H was used as Example 1.

Example 2

8 Parts of the electroconductivity-controlling agent B prepared in Example B-2 were added to 1000 parts of the previously prepared electrodeposition coating composition G to adjust the electroconductivity of the composition to 1300 μS/cm to give electrodeposition coating composition I. The electrodeposition coating composition I was used as Example 2.

Example 3

3 Parts of the electroconductivity-controlling agent C prepared in Example C-3 were added to 1000 parts of the previously prepared electrodeposition coating composition G to adjust the electroconductivity of the composition to 1100 μS/cm to give electrodeposition coating composition J. The electrodeposition coating composition J was used as Example 3.

Example 4

400 Parts of ion-exchanged water were added to 1000 parts of the previously prepared electrodeposition coating composition G to reduce the solid content (7%) to 5%. This procedure reduced the electroconductivity of the composition (890 μS/cm) to 640 μS/cm. 8 Parts of the electroconductivity-controlling agent A prepared in Example A-2 were added to the composition to adjust the electroconductivity of the composition to 1100 μS/cm to give electrodeposition coating composition K. The electrodeposition coating composition K was used as Example 4.

Comparative Example 3

1 Part of the electroconductivity-controlling agent D prepared in Comparative Example D was added to 1000 parts of the previously prepared electrodeposition coating composition G to adjust the electroconductivity of the composition to 1200 μS/cm to give electrodeposition coating composition L. The electrodeposition coating composition L was used as Comparative Example 3.

Cationic electrodeposition coating compositions prepared in Examples and Comparative Examples and cured cationic electrodeposition coatings therewith were evaluated according to the following methods.

Throwing Power

Throwing power of the cationic electrodeposition coating composition is evaluated by a so-called 4 BOX method. Specifically, as shown in FIG. 1, four steel plates 11 to 14 (JIS G 3141 SPCC-SD) treated with zinc phosphate (SURFDINE SD-5000 available from Nippon Paint K.K.) were placed vertically in parallel at 20 mm interval, and lower parts of the both side planes and the bottom plane were covered by an insulating material such as an adhesive cloth tape to prepare a box 10. The steel plates 11 to 13, except plate 14, had an opening 15 of 8 mm phi on the lower side.

4 litter of the cationic electrodeposition coating composition was filled into a vinyl chloride vessel to obtain a first electrodeposition bath. As shown in FIG. 2, the box 10, as an article to be coated, was dipped in the electrodeposition vessel 20 filled with the electrodeposition coating composition 21. Herein, the coating composition 21 penetrated inside of the box 10 only through the opening 15.

The coating composition 21 was stirred by a magnetic stirrer (not indicated in FIG. 2). The steel plates 11 to 14 were electrically connected, and a counter electrode 22 was placed at the position of 150 mm distant from the nearest steel plate 11. A voltage was applied between the steel plates 11 to 14 used as a cathode, and the counter electrode 22 used as an anode, thereby the steel plates were subjected to the cationic electrodeposition coating. The voltage was increased for 5 seconds after its application to allow thickness of the coating formed on surface A of the steel plate 11 to be 15 μm. Then, the conventional electrodeposition procedure was took place wherein the applied voltage was maintained for 175 seconds, or for 115 seconds in the case of a brief electrodeposition.

After electrodeposition coating, these steel plates were rinsed with water, the coating was cured at 170° C. for 25 minutes, and then cooled in air. Thickness of the coating formed on surface A of the steel plate 11 nearest from the counter electrode 22 was measured. Then, thickness of the coating formed on surface G of the steel plate 14 farthest from the counter electrode 22 was measured. Throwing power of the cationic electrodeposition coating composition was evaluated with a ratio: thickness of the coating on surface G/thickness of the coating on surface A (ratio G/A). Evaluation bases are as follows.

-   Excellent: G/A>50% (indicated as lank A) -   Bad: G/A=50% or G/A<50% (indicated as lank B)

Compatibility to Zinc-Steel Plate

A voltage was elevated to 220 V within 5 seconds and applied for 175 seconds to a chemically treated galvanized steel plate, which had been prepared by alloying and melting to give an electrodeposition coating thereon. The plate was rinsed with water, and then baked at 170° C. for 25 minutes. The resulted coating was observed and evaluated as follows.

-   Lank A: Excellent conditions having no defects on the coating -   Lank B: No good conditions having slight defects on the coating -   Lank C: Bad conditions having significant defects on the coating

Appearance of Horizontal Face

A steel plate was set horizontal in a cationic electrodeposition coating composition, without stirring, and then electrically deposited and baked to give a plate having a cured electrodeposition coating thereon. The appearance of the coating was visually evaluated as follows.

-   Lank A: Excellent appearance having no problems -   Lank B: No good appearance having rough texture due to slight     precipitation of pigment -   Lank C: Bad appearance having precipitation of pigment

Electroconductivity

The electroconductivity of the cationic electrodeposition coating composition prepared in each of the Examples and Comparative Examples was measured with a electroconductivity meter (CM-305 available from DKK-TOA CORPORATION) under the conditions that the temperature of the liquid composition was 25° C.

TABLE 1 Examples 1 2 3 4 Electroconductivity- A B C A controlling agent MEQ(B) of 340 390 450 340 electroconductivity- controlling agent (mmol/100 g) Molecular weight of 2100 1000 9800 2100 electroconductivity- controlling agent Neutralizing acid formic sulfamic lactic formic acid acid acid acid Solid content of 7 7 7 5 coating composition (%) Pigment content (%) 5 5 5 5 Electroconductivity 1200 1300 1100 1100 of coating composition (μS/cm) Throwing power A A A A Compatibility to A A A A Zinc-steel plate Appearance of A A A A horizontal face Comparative Examples 1 2 3 Electroconductivity- none none D controlling agent MEQ(B) of — — 740 electroconductivity- controlling agent (mmol/100 g) molecular weight of — — 89 electroconductivity- controlling agent Neutralizing acid none none formic acid Solid content of 20 7 7 coating composition (%) Pigment content (%) 23 5 5 Electroconductivity 1600 890 1200 of coating composition (μS/cm) Throwing power A B B Compatibility to A A C Zinc-steel plate Appearance of C A A horizontal face

Regarding the cationic electrodeposition coating compositions of Examples 1 to 4, each of which contains the present electroconductivity-controlling agent, the electroconductivity of the composition is in an appropriate range. Therefore, these compositions provide no insufficiencies in throwing power and coating appearance. The cationic electrodeposition coating composition of Comparative Example 1 has a conventional solid content, i.e., 20 wt %. The electroconductivity of the composition is in the range defined in the present invention. The solid content of the composition, however, is higher, and therefore the composition provides an inferior appearance of horizontal face. The cationic electrodeposition coating composition of Comparative Example 2 has 7 wt % of solid content, which is a lower solid content type. The electroconductivity of the electrodeposition coating composition is insufficient, and therefore throwing power is low. Regarding the cationic electrodeposition coating composition of Comparative Example 3, wherein the amino group-containing compound has been introduced into the cationic electrodeposition coating composition of Comparative Example 2, the amine value of the amino group-containing compound is deviated from the range defined in the present invention. Both of the throwing power and the compatibility to the zinc-steel plate of the composition are inferior. 

1. An electroconductivity-controlling agent used for a low solid content type cationic electrodeposition coating composition having 0.5 to 9.0 wt % of solid content of the coating composition, which agent includes an amino group-containing compound having 500 to 20000 of molecular weight and 200 to 500 mmol/100 g of amine value, to adjust an electroconductivity of the composition to 900 to 2000 μS/cm.
 2. The electroconductivity-controlling agent according to claim 1, wherein the amino group-containing compound is an amine modified epoxy resin or an amine modified acryl resin.
 3. The electroconductivity-controlling agent according to claim 2, wherein the amine modified epoxy resin is an epoxy resin wherein an epoxy group has been modified with an amine compound.
 4. The electroconductivity-controlling agent according to claim 2, wherein the amine modified acryl resin is an acryl resin having an epoxy group modified with an amine compound.
 5. The electroconductivity-controlling agent according to claim 3, wherein the epoxy resin is bisphenol type, t-butylcatechol type, phenol novolak type or cresol novolak type.
 6. A low solid content type cationic electrodeposition coating composition having 0.5 to 9.0 wt % of solid content of the coating composition, which includes an electroconductivity-controlling agent comprising an amino group-containing compound having 200 to 500 mmol/100 g of amine value, and which composition has 900 to 2000 μS/cm of electroconductivity.
 7. A method for adjusting an electroconductivity of a cationic electrodeposition coating composition, which includes steps of: adding an electroconductivity-controlling agent to a low solid content type cationic electrodeposition coating composition having 0.5 to 9.0 wt % of solid content of the coating composition, and adjusting an electroconductivity of the cationic electrodeposition coating composition to 900 to 2000 μS/cm during the above step, wherein the electroconductivity-controlling agent includes an amino group-containing compound having 200 to 500 mmol/100 g of amine value.
 8. A method for supplying an electroconductivity-controlling agent to a cationic electrodeposition coating composition, which includes steps of: supplying an electroconductivity-controlling agent to a low solid content type cationic electrodeposition coating composition having 0.5 to 9.0 wt % of solid content of the coating composition, and adjusting an electroconductivity of the cationic electrodeposition coating composition to 900 to 2000 μS/cm during the above step, wherein the electroconductivity-controlling agent includes an amino group-containing compound having 200 to 500 mmol/100 g of amine value. 